The present invention relates, in general, to the control of the internal atmosphere in equipment rooms. It is specifically concerned with the temperature control in electrical and telecommunications equipment cabinets located in environments where cooling is required to ensure reliable operation.
Maintaining the temperature of the air within specified limits inside electrical and telecommunications cabinets, which will below be referred to by the more generic term xe2x80x9cequipment cabinetsxe2x80x9d, such as outdoor equipment cabinets is important in order to ensure normal operation. An excessively high indoor air temperature will shorten the service life of equipment and may cause malfunctions. There exists a maximum permissible temperature for air inside outdoor installation cabinets containing telecommunications equipment, and if this temperature limit is exceeded, the equipment will be switched off to avoid damage.
Base transceiver stations for mobile communications networks are increasingly being installed out of doors in locations such as roofs, walls and, where possible, on the ground. The need for equipment cabinets suitable for outdoor installation is evident.
The heat generated by the equipment inside the cabinet essentially affects the air temperature inside the cabinet. In the case of a base transceiver station, the components that produce most heat are the transmitters on the transmitter/receiver units (TRX units). Other major sources of heat are the power supply unit and various output stages. Where the amount of heat being generated is not very high, gravity air circulation is enough to dissipate the heat. This is a suitable solution for geographical areas where the outdoor temperature does not reach a high level. Where the outdoor air temperature is high, forced air circulation may be used. This is accomplished by drawing ambient-temperature air from outside the equipment cabinet, routing it through a filter and sucking the filtered air with a fan into the cabinet. The air passes through the cabinet, absorbing heat generated by the components, and the warmed-up air is blown out of the cabinet. Forced air circulation can be used to prevent the air temperature inside the cabinet from exceeding the outdoor temperature.
The advantage of gravity and forced air circulation lies in its low cost and simplicity. The drawbacks include the ingress of moisture, small particles and gaseous impurities into the equipment cabinet. Where the temperature and/or humidity of the ambient air fluctuates greatly, as in tropical conditions, for example, humidity inside the cabinet condenses. Typically, condensation occurs when the outdoor temperature falls in the evening and at night; this causes the temperature of the air inside the cabinet to drop, resulting in a decreased capacity to contain moisture. Condensation on surfaces may adversely affect the operation of electrical equipment. Moreover, the units inside the cabinet must be encased to prevent contamination, which complicates maintenance and servicing. In areas with a lot of dust, filters tend to clog quickly and must be replaced frequently. In cold climates, the filters can freeze over and be blocked by snow.
However, large volumes of heat cannot be removed from a closed space without equipment performing mechanical work, such as air-to-air heat exchangers, air-to-liquid heat exchangers or heat-pipe heat exchangers that are all here collectively referred to as heat exchangers. Equipment cabinets fitted with heat exchangers are designed to operate in the medium temperature range. Medium temperature refers to outdoor air temperatures up to +40xc2x0 C.
The advantages of using a heat exchanger include its technical reliability and the fact that cabinets can be sealed and weatherproofed to facilitate impurity and humidity control. The drawback is that because the temperature difference between the heat exchanger inlet and outlet side is about 15xc2x0 C., the internal temperature of electronic components may, at high outdoor temperatures, easily exceed the maximum permissible limits. For this reason, equipment cabinets fitted solely with heat exchangers cannot be used when the ambient temperature exceeds +40 C.
A third known method of controlling temperature is to use an air-conditioning unit. This incorporates a cooling unit containing liquids, such as Freon, that evaporate at a low temperature to maintain the temperature inside the cabinet at a suitable level even if the ambient temperature exceeds 40xc2x0 C. Humidity can be controlled and the equipment cabinet can be completely sealed. The disadvantage is that the air-conditioning unit is an expensive piece of technical equipment that requires servicing and consumes a lot of energy.
Publication WO 96/19046 suggests that the equipment cabinet be divided into two compartments that are isolated from each other by an airtight wall. The first compartment houses equipment that generates a lot of heat, such as transmitters that are encased in housings fitted with cooling fins. The second compartment houses equipment that produces little heat. The first compartment is cooled using forced air circulation by drawing air from the outside and blowing it across the cooling surfaces of the housings out of the cabinet. The second compartment is cooled by an air-to-air heat exchanger located in the first compartment. Thus, the forced air circulation in the first compartment cools the heat exchanger, so that heat is transferred from the closed air circuit of the second compartment, which incorporates a fan, to the first compartment and then outside via the heat exchanger.
The solution presented in the said WO Publication permits the operation of the equipment cabinet even at high outdoor temperatures because components generating a lot of heat are cooled by the forced circulation of air drawn from outside. The disadvantage is that the equipment must be carefully encased and sealed to prevent contamination and to improve the efficiency of heat transfer. An equipment cabinet that is split into two compartments allowing for the provision of separate cooling systems for the compartments is an expensive special design. Placing the telecommunications components in two compartments within the cabinet and installing the cabling between the components in separate compartments also increase costs.
The objective of the present invention is to provide a multi-purpose cooling system for an equipment cabinet that permits the operation of telecommunications equipment over a wide temperature range and, specifically, at extremely high outdoor temperatures common around the equator. However, the cooling system of an equipment cabinet should be as simple as possible and made of standard components and, preferably, such that no special casings around the components are required.
The present invention is based on the observation that even in the hottest regions the period during which extremely high temperatures of over +40xc2x0 C. or even over +50xc2x0 C. occur is relatively short and that most of the time the temperature remains below +40xc2x0 C. Thus, the cooling system can be bipartite with the first section consisting of a cooling system with a closed circulation of the air inside the cabinet and the second section consisting of a cooling system with an open circulation of the air inside the cabinet.
The cooling system utilizes a control logic that directs the first set of cooling equipment alone to cool the air inside the cabinet at temperatures within a temperature range below a certain predefined outdoor temperature. The control logic switches on the second set of cooling equipment when a predefined outdoor temperature is reached.
Consequently, for most of the year, the cooling of the cabinet can be effected using the first set of cooling equipment that incorporates an air-to-air heat exchanger. During the short period of time when the heat exchanger cannot cool the equipment cabinet efficiently enough, the second set of cooling equipment with forced flow-through air circulation will be engaged. For cooling, the second set of equipment uses filtered outdoor air without processing it in any way. Because this period of time is short it is not necessary to encase the equipment and the risk of the filters getting clogged is so low that they need not be replaced often. By optimising the operation of the heat exchanger and through-flow air circulation according to the geographical area where the equipment cabinet is located and by giving due consideration to the specific conditions prevailing at the installation site, such as wind, the sun exposure/shadow ratio, etc., it is possible to achieve the best possible performance.
Tapping air directly from the heat exchanger and/or the through-flow air circulation inlet by means of tubing can further enhance the cooling of selected pieces of equipment. Selected equipment can also be heated by placing a heating resistor in their vicinity. Installing heat sources close to critical components and using them to warm up such components near the operating temperature facilitates, among other things, the cold start of a base transceiver station. This is necessary if the outdoor air temperature at the cabinet installation site falls below zero.